Metal pipes are used worldwide for the transportation of, for example, petroleum, petroleum products, gas, water and solids from the place where they are found or produced to the consumer. Metal pipes must be protected on the one hand against corrosion and on the other hand against mechanical defects, in order to ensure the operational reliability of the lines over a long period. Exterior coatings of metal pipes must possess a high impact strength, compressive strength and abrasion resistance, and it is necessary to prevent damage to the exterior coatings during the transportation, storage, laying and operation of the metal pipes. In addition, exterior coating compositions for metal pipes should have a good aging resistance, good weathering resistance during the interim storage of the pipes, an outstanding resistance to external influences, good dielectric properties and a low permeability to water vapor and to gases.
One-coat systems based on epoxy resin are known for the exterior coating of metal pipes, but because of the low mechanical strength of the epoxy resin coatings these systems are disadvantageous when subjected to impact stress.
It is also known to coat the exterior of metal pipes using a three-layer method which involves first applying a powder primer based on epoxy resin to the metal substrate, applying a hot melt adhesive to the powder primer, and subsequently applying a polyolefin cladding. Three-layer coatings of this kind for metal pipes are described in, for example, EP-A-57 823, GB 1 542 333, DE-A-22 22 911, DE-A-22 57 135, DE-A-19 65 802, DE-A-29 44 809 and DE-A-32 30 955. These three-coat systems comprising epoxy resin primer, thermoplastic hard adhesive and polyolefin cladding material go a long way to fulfilling many of the requirements. They are markedly superior to the one-coat systems. The three-coat systems combine the positive properties of the thermosetting primer layer (good adhesion to steel) with the thermoplastic polyolefin cladding (toughness, long-term resistance, good dielectric properties, low gas permeability).
The use of powder coatings based on epoxy resins, dicyandiamide curing agents and crystalline or amorphous silicic acids as powder primer in the three-layer exterior coating compositions for metal pipes is known. Also known are powder primers comprising epoxy resins, phenolic curing agents and amorphous silicic acid fillers.
However, the disadvantages of the three-layer systems known up to now consist in an inadequate peel strength after storage in hot water, in which partial detachment of the powder coating from the substrate occurs during peeling after storage in hot water. Furthermore, the properties with regard to submigration during cathodic delamination are a continual target for improvement.
The object of the present invention was therefore to provide multi-layer coatings for the coating of metal pipes, which coatings do not have the disadvantages described above. The intention was thus to increase, in particular, the peel resistance of the coatings after storage in hot water, and also to obtain excellent results in respect of cathodic delamination. The coatings were to be firmly adhering and resistant and were to provide a high degree of protection against corrosion.